This book hits the ground running and never slows down. Definitely the best in the series. The plot is the most convincing (no magic technology), and Carter loses some little writing tics she'd picked up in the course of writing the previous five. Characters don't "cut their eyes" at anyone here. No one "cocks her hip." And there aren't. Any. Sentences. Like. This. None of those are unforgivable quirks, but Carter was using them a bit too liberally before she cleaned up her act for "United We Spy."
So much happens and so many loose ends are tied up that anything I say about the plot would be a spoiler; so I'll just suggest that it's definitely worth taking the time to start with the first book in the series and read on from there. (If you're a reasonably fast reader, you'll probably burn through these books at a rate of about one a day, anyway. So it's not like I'm asking you to make a serious commitment.) Sure, Carter does the work to make this technically a stand-alone; but why would you want to read what's probably the last book in the series *first*?
I teared up toward the end. But I've been in bed with a bad cold for three days now, so take it for what it's worth.
Carter is influenced and inspired by the Buffy and Harry Potter universes. She mentions Buffy by name in at least one of the books, and come on -- a boarding school where the students learn the equivalent of magic skills that they have to keep secret from the rest of the world? But Carter's writing and the world she's created here are strong and completely her own. Get the Gallagher Girls books for your teen reader (my son loves them) and then sneak them to read yourself. You won't be sorry.